In the image sensor art there is a continual drive towards increasing the resolution of the image sensor so that in turn higher resolution hard and soft copies of the sensed images can be formed. One of the techniques that has received wide spread use in the digital image processing industry involves a process called dithering. The term was first used to define a periodic controlled jitter type motion that was applied to servo motors to decrease the stiction effect between mechanical pieces. In the image processing art the term is used to describe the movement of an image carrying beam from a reference scanning path for a first pass over the image sensor to a displaced path for a second pass. The displacement between the two paths is fixed such that the portion of the image, formed in the beam, that impinges onto the spacing between each of the photosensitive elements that form the sensor array on the first pass impinges on the photosensitive elements on the second or subsequent passes. The process of dithering is thus employed to increase the effective resolution of an image sensor. Another way to conceptualize the basic process is one wherein a small single sensor is moved to different positions on an image thereby collecting data at each position. Such a process is shown pictorially in FIG. 1. This process can be extended to image sensors which are composed of a regular matrix array of individual photosensitive elements. In a typical two-dimensional dithering application, each individual sensor in a matrix array is employed to obtain information at each of four non-overlapping adjacent picture elements (pixels). Since four times as much information is gathered as there are elements in the imager, the effective resolution of the imager is multiplied by four.
Two-dimensional dithering is performed straightforwardly when the photosensitive area of the sensor elements used to acquire each pixel of an image are square and the imager has a fill factor of (100%)/n.sup.2 where n is an integer. For example, when n=2, the sensor has a fill factor of 25%, and non-overlapping adjacent pixel locations are readily obtained in the image with two-dimensional dithering.
When one desires to increase the resolution of an imager possessing either a high fill factor and/or non-square sensor elements, one typically masks off a portion of the photosensitive area in each of the individual sensor elements to achieve the proper fill factor and a square aspect ratio. This is illustrated in FIG. 2a and 2b. One consequence of this masking is a reduction in the imager sensitivity proportional to the percentage of the masked area of the sensors. This is especially of concern for CCD imagers which typically exhibit a low sensitivity in the blue (400-500 nM) part of the visible spectrum.
In U.S. Pat. No. 4,710,803 entitled "Color Filter and Color Image Sensor Using the Same" by Suzuki et al. there is disclosed a color sensor array wherein the photosensitive elements representing a pixel element are covered with three L-shaped filter masks representing the primary colors R, G, and B. This configuration is urged for saving electrical wiring and for providing a more consistent exposure of each of the individual filter masked elements to the same portion of the original image.
Another patent of interest for its teaching in this area is U.S. Pat. No. 4,481,530 entitled "Color Filter Arrangement for Optoelectric Converting Device" by Wagensonner et al. In that patent it is recognized that the area of a photosensitive element assigned to collect the green components of an image has to be different in size than the area assigned for the blue and red and in a like manner the red area has to be different in size from the blue. The main reason for this difference is that the photosensitive elements forming the sensor respond to the different frequencies of light in a different manner, i.e. green light provides a greater electrical output than for example blue light. Another reason for this difference is that the human eye also does not treat each of the colors with the same degree of responsiveness. A color image formed from equally weighted sensor signals would therefor not appear correct to the human eye. Although the referenced patent deals with the responsiveness of the photosensitive material to the frequency of the impinging light by adjusting the area of each sensor according to the color it is to detect, it would also be obvious that the area can be held constant and that the signals from each sensing element could be weighted to provide the same effect.